This invention relates to bus communication method and bus communication system. More particularly, the invention relates to bus communication method and bus communication system for sending data with destination address and source address attached thereto from a source unit to a destination unit via a system bus, wherein connected to the system bus are redundant apparatuses each of which is provided with n number of units (n.gtoreq.2) for working respectively active unit and standby unit and is possible to perform processing continuously by switching over to the standby unit if one of the active units fails.
A system for communicating data between units each other via a system bus to which a plurality of units are connected is well known. FIG. 12A and FIG. 12B are for explaining conventional system bus communication method, in which numeral 1 denotes a system bus, numeral 2 a redundant apparatus which is refered to as apparatus A hereinafter, and numeral 3 a redundant apparatus which is refered to as apparatus B hereinafter. The redundant apparatuses 2,3 have respectively redundant structure and provided with units 2a, 3a of system 0 and units 2b, 3b of system 1. The units 2a, 2b and the units 3a, 3b are all able to work as not only an active unit, but also a standby unit and processing is continued by switching over to the standby unit if the active unit fails.
Data is sent from one of the units in the source redundant apparatus to one of the units in the destination redundant apparatus by attaching destination address DA for specifying destination unit and source address SA for specifying source unit to the data as shown in FIG. 13. Since a inherent unit address is assigned to each unit of system 0 and system 1 in the redundant apparatus, the source unit attaches the unit addresses to the data as a source address and a destination address.
According to the above-mentioned system, each unit in the redundant apparatuses is in active or standby state and the state of the unit is changed in accordance with the condition of system operation. For reasons of this, it is general for a source unit to decide a destination address by recognizing whether each unit in destination redundant apparatus is active or stanby and start communication between units. More specifically, in a case where a source unit sends data to an active unit in the destination redundant apparatus, the source unit must check which unit in the destination redundant apparatus is active. If a unit 3a of system 0 is active, the source unit attaches the unit address of the unit 3a of system 0 as a destination address to data to be sent and sends the data thereafter. For example, assuming that in FIG. 12A 4 address of the unit 2a of system 0 in the apparatus A is "0A00", 2 address of the unit 2b of system 1 is "0A01", 3 address of the unit 3a of system 0 in the apparatus B is "0B00", 4 address of the unit 3b of system 1 is "0B01" and 5 the unit 3a of system 0 is active, the destination address becomes "0B00". On the other hand, if the unit 3b of system 1 is active as shown in FIG. 12B, the destination address becomes "0B01".
In the above, although it is described that the data is sent to the active destination unit, maintainance data and apparatus management data are sent to the standby unit. If the communication data has to be sent to the standby unit, the source unit must check which unit in the destination redundant apparatus is standby. And if a unit of system 0 is standby, the source unit attaches the address of the unit of system 0 to the data as a destination address and if a unit of system 1 is standby, the source unit attaches the address of the unit of system 1 to the data as a destination address and sends the data thereafter.
According to the conventinal communication system, address of each unit in the redundant apparatus is fixed. By reason of this, in a case where data has to be sent to active or stadby unit, it is necessary for the source unit to distinguish which unit among units of system 0 and system 1 in the destination apparatus is active or standby. As a result, there is provided in the communication system a state management mechanism which is composed of a communication means for communicating the state information each other between redundant apparatuses or units, a memory for storing the state of each unit in all apparatus and so on. However, there is problem that the state management mechanism has complex structure which causes high price of the system.